I think part of the disagreement here is that "reasons" has different meanings. A reason can refer to intent (why did you quit your job?) or it can refer to how something came to happen (why won't my car start). It's an important distinction because only conscious beings have intent (it's not like your car is refusing to start because she's pissed she caught you looking at the neighbor's new BMW ).
So, since evolution is not a conscious being (if you want to talk about evolution being an extension of God's will, fine but it's God's intent then, not evolution's and that's a whole other can of worms anyway) and cannot have intent, "why" questions about evolution are better asked as "how" questions. Not "Why does sex feel good" but rather "How it came to be that sex feels good". Evolution explains how it came to be, not why. When sex feels good to an animal, that animal will tend to procreate more often, and over time the animal that procreates more often will come to be more prevalent. That is how sex came to feel good to humans. There is no "why" sex feels good. Evolution didn't intend for sex to feel good; evolution doesn't intend anything. Evolution is just an explanation for how the animals that currently exist came to exist in the state that they do.
It's important to be mindful of this distinction because it avoids burdening questions about what we should do with considerations of what evolution intends us to do - that's nonsensical, we shouldn't care about what evolution "intends" us to do because evolution doesn't intend anything, it can't intend. (on a side note, even if evolution did have an intent, so what? Why should we do what evolution wants? **** her. ). Evolution isn't any sort of guide on what we should do. That's ultimately up to us and to decide according to our own wishes and goals.
Quite frankly I (and most probably most people) don't give two ****s about evolution or spreading my genes. I have sex because I enjoy it. If I ever have kids it's going to be because I want the experience of raising children and because I want to help give another person the experience of life, not because evolution wants me to propagate my genes.